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Fastened mate orientation error/bug/unexpected behaviour

rune_thorsen229rune_thorsen229 Member Posts: 156 EDU
In assembly, when placing a carefully oriented fasten mate between two parts like these (notice the mate orientation)


I would expect this to happen


instead I get this 

It depends on the starting position of the parts, but I consider it as a serious bug. You may argue that it is intended behaviour but it is rather illogical if it is a 'fasten mate' because it behaves like a revolute mate at the moment of solving. 
I would expect a fasten mate to solve to match the orientation, place and plane as initially and only subsequently when the user changes orientation it should turn

Comments

  • rune_thorsen229rune_thorsen229 Member Posts: 156 EDU
    Chatting with Niel I said 'I expected that the Fastened mate would snap to same orientation and keep that.' and  he clarified that there are 8 possible orientations. It would be nice with a mate that you could 'trust' or 'reset' to matching all orientations
    Furthermore I encountered this problem where mates seems to give an unexpected offset

    I use a line in a plane to place the mate 
     and I can't figure out what the problem is.
    Document :https://cad.onshape.com/documents/dd4e620547fb95eb0b416aee/w/811d2761aa092256e78facf8/e/b82448a6521d3b3e1aaf5f93

  • rune_thorsen229rune_thorsen229 Member Posts: 156 EDU
    NeilCooke sorry misspelling
  • owen_sparksowen_sparks Member, Developers Posts: 2,660 PRO
    Hi.

    So we didn't get off to the best of starts, so here is an olive branch. :)

    Looking at your first example you have placed a pair of local coordinate systems (aka mate connectors) onto your parts.

    When we make use of those connectors in a Mate feature they will be brought together and the parts will be transformed such that these axis align.

    By the power of Smints...

    Add mate connectors:-


    Add a mate.

    In this case it is a fastened mate, so there are no degrees of freedom.  Note the orientation of the red axis in my photo and your first screen-grab  This is absolutely the expected outcome of this mate. (Other than the fact that I can't press the tins into overlapping space!)

    A "revolute mate" allows one degree of freedom (within whatever limits you chose) it does not describe the rotation of a fastened mate as the axis are aligned in the mating process.  So the revolute allows a mechanism to rotate one mated.

    Placing mates is aided by using the shift key when on the planar surface to de-clutter inferences.  After it's placed reorientation or rotation can be made to the placement, but often its simpler to place them on the required vertex and then use the flip and rotate quadrant in the mating operation to gain the desired orientation rather than insist on placing the mate connector with each axis pointing in a defined orientation. 

    Creating explicit mates in PartStudios at whatever rotations and offsets you prescribe is a very powerful and hopefully intuitive way of either moving parts around or building up assemblies.  (This is relevant to your second question.  Mates don't just slide parts towards each other.  They align all three axis of their mates.  So the orientation of the mate must be considered.  There several ways to control this, but perhaps that's another topic.)

    In the main in the last 4 years I've been on this forum users tend to love mate connectors and take to them easily.  Movement limits in assemblies can more challenging but Onshape have gone to some lengths to improve things there.

    I don't know if your interpretation of my answer that is in turn an interpretation of your question meets your expected outcome, but either way please don't down-vote this comment, or anyone else trying to help you, without a damn good reason. o:)

    Happy CADing!

    Cheers,
    Owen S.
    Business Systems and Configuration Controller
    HWM-Water Ltd
  • john_mcclaryjohn_mcclary Member, Developers Posts: 2,861 PRO
    Chatting with Niel I said 'I expected that the Fastened mate would snap to same orientation and keep that.' and  he clarified that there are 8 possible orientations. It would be nice with a mate that you could 'trust' or 'reset' to matching all orientations
    Furthermore I encountered this problem where mates seems to give an unexpected offset

    I use a line in a plane to place the mate 
     and I can't figure out what the problem is.
    Document :https://cad.onshape.com/documents/dd4e620547fb95eb0b416aee/w/811d2761aa092256e78facf8/e/b82448a6521d3b3e1aaf5f93

    We do not have permission to access document, please set it to public so we can have a look.

    --------------------
    As for your original topic. In general, the reason this is not a bug is:
    Because each mate connector has 8 possible orientations relative to the origin point of each mate connector.
    I understand your logic, default should be origins of both mate connectors are aligned X-X Y-Y Z-Z.
    But if that were the case, then you will end up editing your mate orientation 99% of the time, rather than just pre-position the part and snap a mate. Much quicker the way Onshape currently has it.

    Basically ignore the mate connector xyz position, treat it like a point, then flip/rotate until it is in the correct position.

    This is the same behavior as I'm used to seeing in SolidWorks (pre-position before mating lest you fold your assembly into the fourth dimension)
    with the exception of mating two origins with a coincident. Those behave as you expect mate connectors to.
  • rune_thorsen229rune_thorsen229 Member Posts: 156 EDU
    I have shared it with Niel and [email protected]
    https://cad.onshape.com/documents/dd4e620547fb95eb0b416aee/w/811d2761aa092256e78facf8/e/b82448a6521d3b3e1aaf5f93
    "I understand your logic, default should be origins of both mate connectors are aligned X-X Y-Y Z-Z."
    Exactly. I spent some time trying to understand why things were not perfectly aligned. Then I realigned the mates on my parts so they should be perfect when assembled just to find that I have to spend time realigning it all again. 
    Most of all what I don't understand is why the solver engine doesn't do that mating as first guess. 
    Secondly its faster selecting the relevant mates from the Instance list.
    Finally I think it's exposes a error prone result when you cannot trust mates to align xxyyzz. I saw a comment thread where somebody are aligning thin sheets having the same probelm.
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